We loved staying at Napa's, her multi-floor apartment is a true gem, you can easily stay there for a month or (much) more. It felt like home the moment we stepped in. The location couldn't be more central or more strategic (Chula, Ari, Pathum Wan, Rama IV, Silom, Sathorn). We lived in Bangkok for 4+ years, so we know Bangkok inside out.
Napa is also super responsive and extremely helpful.